1. Field of the Invention
The present invention relates generally to an irradiation system, and more particularly to a water disinfection system comprising an electrodeless UV lamp with an external drive coil.
2. Description of the Related Art
Conventional water disinfection systems typically include an elongated ultraviolet (UV) lamp which projects UV radiation radially outwardly into a surrounding water flow passage. As the water flows through the flow passage, the UV radiation kills harmful bacteria and microbes in the water. The flow passage typically has an annular cross section, defined on the inside by a transparent sleeve which surrounds the UV lamp, and on the outside by an opaque cylindrical container which contains the water and prevents UV radiation from escaping from the system.
In the conventional water disinfection system, the lamp typically comprises a linear low pressure discharge lamp which includes two electrodes inside a glass envelope. Each electrode typically comprises a tungsten coil coated with the oxide of an alkaline earth element such as barium, strontium, or calcium. The alkaline earth oxide is provided on the tungsten wire to increase the thermionic emission of electrons at the cathode. The lamp is very similar to a conventional fluorescent lamp, except that there are no phosphor coatings, and the glass envelope transmits a substantial amount of the UV radiation.
Glass provides the advantage that the electrical leads to the electrodes can be sealed relatively easily in a gas-tight manner with the glass in a softened state. The glass envelope, however, becomes solarized over time from exposure to the UV flux. Solarization is typically exhibited as a darkening of the envelope and results in increased absorption of UV radiation. After a certain period of use, the glass lamp typically must be replaced due to solarization.
The electrodes also have a relatively short lifetime, because the alkaline earth oxide material is depleted over time, as in a conventional fluorescent lamp, primarily by evaporation. After the alkaline earth oxide is depleted, the lamp will either fail to start or will enter a glow discharge state during operation, which quickly sputters away the remaining cathode material. In addition, the electrodes in such a lamp can support only a limited power output, thus limiting the disinfection capability of the device.
It would be desirable, therefore, to have a water disinfection system which was more robust than conventional systems with a greater lifetime and higher available operating power.
An irradiation system, according to an exemplary embodiment of the invention, comprises a conduit which is transparent to ultraviolet radiation, a chamber having an annular cross section, disposed around the conduit, the chamber containing an ionizable gas, and a coil disposed around the chamber for ionizing the ionizable gas to produce an ultraviolet emission which propagates into the conduit.
The invention also relates to an irradiation method comprising the steps of generating a UV emission in a chamber having a conduit passing through the chamber, the conduit being fluidly isolated from the chamber, and flowing a fluid through the conduit such that the fluid is exposed to the UV emission generated in the chamber.
The invention provides several advantages over conventional UV water disinfection systems. For example, the lamp portion of the system has no electrodes which allows the system to operate at a higher power and over a longer lifetime than conventional water disinfection systems. The electrodeless design of the lamp also provides the advantage that there are no electrical leads passing through the walls of the chamber 130 so that the chamber 130 can be easily made of quartz, which is much less susceptible to solarization and has good UV transmission properties. An additional advantage provided by exemplary embodiments of the invention is the ability to easily install multiple lamps in a system in accordance with the desired power for the system.